Classification of complexometric titrations MCQs With Answer

Complexometric titrations are essential in pharmaceutical analysis for quantifying metal ions using chelating agents like EDTA. Classification of complexometric titrations covers direct, indirect (back), displacement, replacement, masked and demasked titrations, and instrumental end-point methods such as potentiometric and colorimetric detection. B.Pharm students must understand factors affecting titration: pH control, conditional formation constants, indicator selection, masking agents, and standardization of titrants. Practical applications include assay of calcium, magnesium, zinc and impurity profiling in drugs and excipients. This concise, keyword-rich introduction prepares you to master theory, calculations, and method selection for routine and advanced metal analysis. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which titrant is most commonly used in complexometric titrations for pharmaceutical metal analysis?

• EDTA (ethylene diamine tetraacetic acid)
• AgNO3 (silver nitrate)
• KMnO4 (potassium permanganate)
• HCl (hydrochloric acid)

Correct Answer: EDTA (ethylene diamine tetraacetic acid)

Q2. Classification of complexometric titrations includes which of the following types?

• Direct, Back (indirect), Displacement, and Replacement
• Acid-base, Redox, Gravimetric, and Precipitation
• Volumetric, Gravimetric, Electrogravimetric, and Chromatographic
• Turbidimetric, Nephelometric, Spectrophotometric, and Colorimetric

Correct Answer: Direct, Back (indirect), Displacement, and Replacement

Q3. In an EDTA titration, the typical stoichiometry between metal ion (M) and EDTA is:

• 1:1 metal:EDTA complex
• 2:1 metal:EDTA complex
• 1:2 metal:EDTA complex
• 3:2 metal:EDTA complex

Correct Answer: 1:1 metal:EDTA complex

Q4. Which indicator is commonly used for determining the endpoint in titration of Ca2+ and Mg2+ with EDTA by color change?

• Eriochrome Black T
• Phenolphthalein
• Methyl orange
• Bromothymol blue

Correct Answer: Eriochrome Black T

Q5. A masked complexometric titration involves:

• Temporarily suppressing the response of certain metal ions using masking agents
• Using a stronger titrant to displace weaker complexes directly
• Measuring endpoint by potentiometry only
• Adding excess EDTA to precipitate the metal

Correct Answer: Temporarily suppressing the response of certain metal ions using masking agents

Q6. Which of the following is a common masking agent used to mask Fe3+ during complexometric titration?

• Cyanide (CN–)
• Sodium chloride (NaCl)
• Hydrochloric acid (HCl)
• Barium sulfate (BaSO4)

Correct Answer: Cyanide (CN–)

Q7. Conditional formation constant (K’) in complexometry differs from the true formation constant (Kf) because it:

• Accounts for pH and ionic strength affecting complex stability
• Is always larger than Kf
• Ignores protonation equilibria of EDTA
• Only applies to gas-phase complexes

Correct Answer: Accounts for pH and ionic strength affecting complex stability

Q8. Which parameter is most critical to control when titrating metal ions with EDTA to ensure accurate complex formation?

• pH of the solution
• Temperature of the burette only
• Atmospheric pressure
• Volume of pipette used

Correct Answer: pH of the solution

Q9. In back titration (indirect complexometric titration), the analyte is:

• Reacted with an excess of reagent and the unreacted reagent is titrated
• Titrated directly with EDTA until endpoint
• Measured by gravimetry after precipitation
• Estimated by color comparison with standards

Correct Answer: Reacted with an excess of reagent and the unreacted reagent is titrated

Q10. Which detection method gives a sharp, pH-independent endpoint in complexometric titrations and is ideal for automated titration?

• Potentiometric titration using a metal ion-selective electrode
• Visual color change with an organic indicator
• Using starch as an indicator
• Conductometric titration with non-ionic analytes

Correct Answer: Potentiometric titration using a metal ion-selective electrode

Q11. EDTA acts as a chelating agent due to which property?

• Multiple donor atoms allowing hexadentate coordination
• High volatility and low boiling point
• Strong oxidizing power
• Hydrophobic alkyl chains

Correct Answer: Multiple donor atoms allowing hexadentate coordination

Q12. Murexide indicator is particularly useful for titrations involving which metal ion?

• Calcium (Ca2+)
• Magnesium (Mg2+)
• Aluminum (Al3+)
• Uranium (UO2^2+)

Correct Answer: Uranium (UO2^2+)

Q13. Displacement titrations in complexometry are based on:

• Strong metal displacing a weaker metal from its complex
• EDTA being displaced by acetate ions
• Changing temperature to break complexes
• Addition of oxidizing agents to displace metals

Correct Answer: Strong metal displacing a weaker metal from its complex

Q14. Which of the following best describes a demasking agent?

• A reagent that releases a masked metal ion by breaking the masking complex
• An indicator that changes color at the endpoint
• A buffer that maintains pH during titration
• A titrant used to standardize EDTA

Correct Answer: A reagent that releases a masked metal ion by breaking the masking complex

Q15. Why is ammonia–ammonium chloride buffer commonly used in EDTA titration of Ca2+ and Mg2+?

• It maintains pH around 10 where EDTA forms stable complexes with Ca2+ and Mg2+
• It reacts with EDTA to form colored complexes
• It precipitates interfering anions
• It reduces metal ions to their elemental form

Correct Answer: It maintains pH around 10 where EDTA forms stable complexes with Ca2+ and Mg2+

Q16. In pharmaceutical quality control, complexometric titration is commonly used to determine:

• Metal impurities and hardness in water and formulations
• Organic content of active pharmaceutical ingredients
• Viscosity of liquid formulations
• Microbial contamination levels

Correct Answer: Metal impurities and hardness in water and formulations

Q17. Which statement about stepwise formation constants (K1, K2, …) of metal-ligand complexes is true?

• They decrease progressively for successive ligand additions in many systems
• They always increase for each successive ligand addition
• They are unrelated to overall stability constant
• They are equal for all metals with EDTA

Correct Answer: They decrease progressively for successive ligand additions in many systems

Q18. Which factor can cause an apparent shift in the observed endpoint of a complexometric titration?

• Presence of competing metal ions or interfering ligands
• Using distilled water instead of deionized water
• Warming the lab slightly above room temperature
• Using a burette made of glass

Correct Answer: Presence of competing metal ions or interfering ligands

Q19. Xylenol orange is used as an indicator for titrations involving which metal ion group?

• Transition metals such as Fe3+ and Al3+
• Alkali metals like Na+ and K+
• Noble gases in solution
• Halide ions like Cl– and Br–

Correct Answer: Transition metals such as Fe3+ and Al3+

Q20. Which method is appropriate when the analyte forms a very stable complex with EDTA that prevents direct titration?

• Back titration using excess EDTA and titration of remaining EDTA
• Direct titration with lower pH
• Ignoring the stability and titrating anyway
• Using gravimetric separation only

Correct Answer: Back titration using excess EDTA and titration of remaining EDTA

Q21. The formation constant (Kf) indicates:

• The equilibrium tendency for metal and ligand to form a complex
• The speed at which the complex forms
• The color intensity of the complex
• The volatility of the complex

Correct Answer: The equilibrium tendency for metal and ligand to form a complex

Q22. In a mixed metal solution, selective determination of one metal can be achieved by:

• Masking other metals or adjusting pH to favor selective complexation
• Adding excess indicator to hide other metals
• Lowering temperature to freeze out metals
• Using distilled water to dilute the sample

Correct Answer: Masking other metals or adjusting pH to favor selective complexation

Q23. Why is standardization of EDTA solution necessary before use?

• Commercial EDTA solutions may not have exact concentration due to purity or weighing errors
• EDTA is always unstable and decomposes rapidly
• EDTA changes color over time making concentration unknown
• It reacts with glassware and must be standardized

Correct Answer: Commercial EDTA solutions may not have exact concentration due to purity or weighing errors

Q24. Which analytical advantage do complexometric titrations offer in pharmaceutical labs?

• High specificity and relatively low reagent consumption for metal analysis
• They quantify organic impurities directly without preparation
• They replace chromatographic methods for all assays
• They do not require pH control or calibration

Correct Answer: High specificity and relatively low reagent consumption for metal analysis

Q25. Which condition would reduce the effective binding of EDTA to metal ions?

• Low pH where EDTA is protonated and less available to chelate
• High ionic strength buffer that stabilizes complexes
• Adding masking agents that strengthen EDTA binding
• Heating the solution to 80°C for short time

Correct Answer: Low pH where EDTA is protonated and less available to chelate

Q26. Which of the following is NOT a typical end-point detection technique in complexometric titrations?

• Mass spectrometric detection of complexes (routine endpoint)
• Visual indicators that change color
• Potentiometric detection with ion-selective electrodes
• Spectrophotometric detection monitoring absorbance change

Correct Answer: Mass spectrometric detection of complexes (routine endpoint)

Q27. For titration of trace-level metal contaminants in pharmaceuticals, which approach increases sensitivity?

• Preconcentration of analyte and using sensitive spectrophotometric or fluorimetric indicators
• Using less sensitive visual indicators only
• Diluting the sample heavily before titration
• Avoiding buffers and leaving pH uncontrolled

Correct Answer: Preconcentration of analyte and using sensitive spectrophotometric or fluorimetric indicators

Q28. Replacement titration involves:

• Treating an analyte metal complex with another metal that forms a stronger complex, releasing the analyte ion to be titrated
• Replacing EDTA with citrate in the titration
• Swapping the indicator in the middle of titration
• Removing buffer and replacing with distilled water

Correct Answer: Treating an analyte metal complex with another metal that forms a stronger complex, releasing the analyte ion to be titrated

Q29. Which concept explains why some metal-EDTA complexes show different stabilities at different pH values?

• Protonation equilibria of EDTA and metal hydrolysis influence conditional stability
• The color of EDTA changes with pH altering stability
• EDTA evaporates at certain pH values
• Water autoionization blocks complexation at all pH

Correct Answer: Protonation equilibria of EDTA and metal hydrolysis influence conditional stability

Q30. In pharmaceutical assays, which metal is routinely determined together as “hardness” by complexometric titration?

• Calcium and magnesium
• Sodium and potassium
• Iron and copper
• Zinc and manganese

Correct Answer: Calcium and magnesium

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